Traditionally, researchers assumed that infants understand very little about the physical world. With the advent of more sensitive methods, however, investigators have come to realize that even young infants possess expectations about physical events. What is the nature of these early expectations, and how do they develop over time? These questions have been at the core of Baillargeon's research program for the past 20 years. This application seeks to test and extend two accounts recently developed by Baillargeon and her collaborators: the first focuses on how infants use their current physical knowledge to reason about physical events and predict their outcomes (reasoning account); the other account examines how infants attain new knowledge about physical events (learning account). Each account makes several testable predictions that will be investigated in the next grant period. The proposed research builds on experiments conducted in the previous grant periods, and also introduces several new research directions. The proposed experiments will make use of two different methods, the violation-of-expectation (VOE) and the object-manipulation (OM) method. In all, 18 projects are planned, organized into seven, inter-related lines of research. These lines examine: (1) the formation and use of event categories; (2) the acquisition of variables in individual event categories; (3) early competencies in infants' reasoning about events from different categories; (4) cueing infants to reason about a new variable in an event category, through exposure to an event from a different category in which this variable has already been identified; (5) effects of event category knowledge on infants' ability to detect surreptitious changes in variable information; (6) teaching infants (in or out of the laboratory) a new variable in an event category, through exposure to appropriate events from the category; and finally (7) the formation and use of abstract object categories, namely, inert and self-moving objects. The proposed research will help us better understand how infants reason and learn about physical events, and as such will give us a conceptually richer and more detailed picture of this facet of cognitive development in infancy.